Doppler Effect
Year of Discovery: 1848
What Is It? Sound- and light-wave frequencies shift higher or lower depending on whether the source is moving toward or away from the observer.
Who Discovered It? Christian Doppler
Why Is This One of the 100 Greatest?
The Doppler Effect is one of the most powerful and important concepts ever discovered for astronomy. This discovery allowed scientists to measure the speed and direction of
stars and galaxies many millions of light years away. It unlocked mysteries of distant galaxies and stars and led to the discovery of dark matter and of the actual age and motion of the
universe. Doppler’s discovery has been used in research efforts of a dozen scientific fields.
Few single concepts have ever proved more useful. Doppler’s discovery is considered to be
so fundamental to science that it is included in virtually all middle and high school basic
science courses.
How Was It Discovered?
Austrian-born Christian Doppler was a struggling mathematics teacher—struggling
both because he was too hard on his students and earned the wrath of parents and administrators and because he wanted to fully understand the geometry and mathematical concepts
he taught. He drifted in and out of teaching positions through the 1820s and 1830s as he
passed through his twenties and thirties. Doppler was lucky to land a math teaching slot at
Vienna Polytechnic Institute in 1838.
By the late 1830s, trains capable of speeds in excess of 30 mph were dashing across the
countryside. These trains made a sound phenomenon noticeable for the first time. Never before had humans traveled faster than the slow trot of a horse. Trains allowed people to notice
the effect of an object’s movement on the sounds that object produced.
Doppler intently watched trains pass and began to theorize about what caused the
sound shifts he observed. By 1843 Doppler had expanded his ideas to include light waves
and developed a general theory that claimed that an object’s movement either increased or
decreased the frequency of sound and light it produced as measured by a stationary observer. Doppler claimed that this shift could explain the red and blue tinge to the light of distant twin stars. (The twin circling toward Earth would have its light shifted to a higher
frequency—toward blue. The other, circling away, would shift lower, toward red.)
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